Trying to determine if car was geared already....

Discussion in '96-04 - 4V Specific' started by SnakeMedic, Sep 15, 2014.

  1. SnakeMedic

    SnakeMedic Member

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    Been thinking about throwing in some 4.10s in my Cobra, but since I have no records of mods on this car I am not sure if it was changed already.

    Found a chart that says at 65 mph in 5th gear what RPM I should be turning for certain gears but my problem is that I know my speedo is off, which leads me to believe it might already be geared. The faster I go, the more it is off and at my perceived 75 mph is when I am actually going 65. Tires/wheels are stock size, so I know that is not the issue.

    Anyone have any insight on how to figure this out? Can I just base it off of RPM and being in 5th gear?

    I know 3.55s turn 2k at 65 mph in 5th gear, so despite not knowing the real speed I could conceivably just use RPM...... I think....

    Any help is appreciated. Though I have owned many Mustangs, I never geared any of them so this is new to me. :undecided:
     
  2. CC'S95GT

    CC'S95GT Legend

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    The only sure way to know is to pull the diff cover.
    Another semi accurate way is to jack the rear end off the ground and place on jack stands. Trans in neutral. Mark drive shaft with sharpie, crayon, or what ever you got handy. Now slowly spin BOTH TIRES TOGETHER 1 revolution and count how many times the drive shaft spins.
    3 1/2 revolutions= 3:55. 3 3/4 revolutions= 3:73, just over 4 revolutions = 4:10 gears.
     
  3. SnakeMedic

    SnakeMedic Member

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    Gotcha. Thanks for the info. If I pulled the diff cover, what exactly would I be looking for?

    Oh, also when I have 2k RPM in 5th gear my speedo shows 70 mph... Ugh.
     
  4. CC'S95GT

    CC'S95GT Legend

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    the ratio will be stamped on the end of the pinion.
     
  5. SnakeMedic

    SnakeMedic Member

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    Perfect. Gracias, amigo.

    Either way, there is no way this has 4.10s in it. I am tempted to just get the 4.10s and find out when I install those. :grin:
     
  6. lutter94

    lutter94 Well-Known Member

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    I am going to guess 3.73's. Post when you verify what it really is......The ford racing speed cal is worth its weight in gold.
     
  7. SnakeMedic

    SnakeMedic Member

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    At least if they are not stock I can always sell them!
     
  8. martinbaca18

    martinbaca18 New Member

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    There is a speedo app on the iPhone. And they are pretty accurate. Just use that to get your mph from that app and then look at your rpms. That's how I would do it.
     
  9. SnakeMedic

    SnakeMedic Member

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    I has no iPhone. Android. But I have used 2 different apps and both are right on with how much I am off. No worries, I am always slower than the speedo so at least I won't get a ticket for being too fast.

    Why have I not ordered the gears yet? Weird.